1. Field of the Invention
The present invention relates to a family of heterogeneous spiro compounds and to elements for organic light emitting devices (OLEDs) in which a heterogeneous spiro compound is used in the emissive layer and/or one or more of the charge transport layers, or as a host material for one or more of such layers.
2. Description of the Related Art
Organic light emitting devices (OLEDs) typically comprise a layer of emissive material between an anode and a cathode. When a bias is applied across the electrodes, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the emissive layer. The holes and the electrons form excitons in the emissive layer to emit light.
Electrodes are chosen to facilitate charge injection. A transparent indium-tin-oxide (ITO) anode has a relatively high work function and is therefore suitable for use as a hole injection electrode, while low work function metals such as Al, Mg and Ca are suitable for injection of electrons and are therefore a typical choice for the cathode.
To improve the power efficiency of an OLED, it is frequently desirable to enhance charge injection at the electrode interface. Hole transport layers and electron transport layers may be added adjacent the respective electrodes to facilitate charge transfer. Depending upon whether hole transport or electron transport is favored, the light emissive layer may be located closer to the anode or the cathode. In some instances, the emissive layer is located within the hole transport or electron transport layer.
Improved performance can be obtained if blocking layers are provided to block against the injection of either holes or electrons from the adjoining layer and their subsequent escape from the device. Likewise, a modifying layer may be used to improve the contact with one or both of the electrodes, or to improve the interface between two other layers.
Some of these layers can be combined. For example, a double-layered structure is fabricated from a combined hole-injecting and transporting layer together with a combined electron-transporting and light-emitting layer. Likewise, a triple-layered structure is composed of a hole-injecting and transporting layer, a light-emitting layer, and an electron-injecting and transporting layer.
Hole transport layers may include triarylamine-based materials, although many other hole transport materials are known. Likewise, an aluminum quinolinolate complex known as AlQ3 is a well known electron-transport material which has been used in OLEDs, although other electron transport materials are known.
Emissive materials having widely varied structures are known in the art and are generally selected based on color, brightness, efficiency and lifetime. These emissive materials may themselves also have electron transport or hole transport characteristics.
In addition, it is possible to form these layers from a “host” material doped with another material (the “guest” material) designed to achieve the desired effect of the layer (for example, to achieve a hole transport effect, an electron transport effect, or an emissive effect). In the case of an emissive guest-host system, the host must be able to transfer energy to the guest so that a maximum amount of energy contributes to emission by the guest rather than being absorbed by the host.
Anthracene and fluorene both have emissive characteristics, and OLED materials based on these compounds have been described in the literature. Spiro compounds comprising an acridine moiety (acridine has the structure of anthracene except that the 10 position is occupied by a nitrogen atom) are also known to have chemiluminescent properties. Likewise, certain xanthene-based compounds have been shown to have luminescent properties (xanthene has the structure of anthracene except that the 10 position is occupied by an oxygen atom.) Spiro bis-fluorene compounds have been disclosed for use in OLEDs. However, there continues to be a need in the art for spiro compounds in which heterogeneous groups are bridged via the spiro atom to impart novel energy transfer, charge transfer and/or emissive properties to the compound overall. It would likewise be desirable to develop “small” (i.e. non-polymeric) compounds having the aforesaid structural characteristics.
There continues to be a need for OLED materials exhibiting thermal stability, having bright, high purity luminescent emission, and for materials which contribute to greater luminescence per injected charge. There continues to be a need for OLED materials having color “tunability,” which is the ability to modify the emissive output of a compound by attachment of substituent groups having absorptive or emissive characteristics.